Hatcher Pass Avalanche Center

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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Sat, April 17th, 2021 - 7:00AM
Expires
Sun, April 18th, 2021 - 7:00AM
Forecaster
Allie Barker
The Bottom Line

The avalanche danger will be MODERATE in the morning for PERSISTENT SLAB and LOW rising to CONSIDERABLE in the afternoon for WET SLAB and WET LOOSE. In case you missed it, the Easter storm came and went, with many avalanches crossing and closing the road. Sub zero temperatures are also a thing of the past. Spring has sprung and so has the emergence of a wet-slab cycle at low elevation. It barely froze at 3500′ last night and it will reach a high of 40-45ºF up to 4500′ the next several days. If you are chomping at the bit to get out and ski or ride once the road is cleared be aware that avalanche danger will continue to remain steady or increase as temperatures rise over the next several days.

Avalanche mitigation occurred on Friday with no results. Road clearing began on Friday and will continue today.

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Sat, April 17th, 2021
Upper Elevation
Above 3,500'
Moderate (2)
Avalanche risk
Mid Elevation
2,500'-3,500'
Moderate (2)
Avalanche risk
Low Elevation
Below 2,500'
Considerable (3)
Avalanche risk
Upper Elevation
Above 3,500'
Avalanche risk
Moderate (2)
Mid Elevation
2,500'-3,500'
Avalanche risk
Moderate (2)
Low Elevation
Below 2,500'
Avalanche risk
Considerable (3)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
Recent Avalanches

All the avalanches pictured below (except this first one)occurred in the afternoon on Friday, April 16th at approx. 1200′ on S and SE aspects. No other avalanches were observed or reported at mid or upper elevation this week.

Numerous wet loose avalanches were also observed on the S/SW aspect of Idaho Peak up to 3500′ on 4/16.

Road cut , AM 4/16

 

Road cut, PM 4/16 Wet loose triggers wet slab and takes out the metal fence panels.

 

4/16 Avalanche #1 near the Avalanche Gate at Gateway Natural Wet Loose triggered Wet-Slab that almost hits the road. 1200′ S/SE aspect .

 

4/16 Avalanche #2 near the Avalanche Gate at Gateway Natural Wet Loose triggered Wet-Slab that almost hits the road. 1200′ S/SE aspect

 

4/16 Avalanche #2 and #3 near the Avalanche Gate at Gateway Natural Wet Loose triggered Wet-Slab that almost hits the road. 1200′ S/SE aspect

Avalanche Problem 1
  • Wet Slab
    Wet Slab
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Slab
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Wet slabs will be a unlikely in the morning, RISING to CONSIDERABLE in the afternoon and likely to human trigger, on East, Southeast, South, and Southwest aspects, at LOW elevation, below 2500′,on slopes above 35º. Naturals will be unlikely in the morning and possible in the afternoon. It will also be possible for wet-loose avalanches to triggers wet-slab avalanches, as seen Friday at 1200′ on the road cut above the Raney homestead, and with 3 other additional wet-slabs at 1200′ near the avalanche gate at the entrance to Hatcher Pass.

Low elevation has reached a tipping point as a result of a sudden increase in temperatures this week. Snow surfaces are moist to wet on E to SW aspects below 2500′. Temperatures at 2700′ have hovered at or above freezing for 3 nights in a row. 3 nights above freezing is our general rule of thumb for predicting wet slabs combined with other factors.  We are not expecting it to freeze below 3000′ for the next several days.  Wet loose avalanches are predictable with time of day, but wet slabs are trickier to predict and follow long periods of above freezing temperatures and can be large in size.

Outlook: Temperatures are forecasted to rise throughout the weekend and into early next week. If it does NOT freeze for 3 nights in a row at 3000′ expect wet-slabs to be possible to likely at mid-elevation, or up to 3500′. We recommend using conservative decision making and widening your safety margin in and around southerly aspects this weekend.

 

Natural wet-loose that triggered a wet-slab at the road cut. 4/16 1200′ S/SE aspect. The debris took out the metal fencing.  This is an indicator of what to expect at mid elevation in the coming days to week with an increase in temperature.

 

Wet-slab avalanche near the avalanche gate, gateway to Hatcher Pass. This small path appears to have run 400 ft , making it close to the road. HPAC has never seen this path run this far before. 4/16 S/SE 1200′

 

 

 

 

Avalanche Problem 2
  • Wet Loose
    Wet Loose
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Wet loose avalanches will be unlikely to human trigger in the morning, and likely to human trigger in the afternoon on E to S to SW aspects below 3500′, on steep slopes above 40º. Naturals will be possible in the afternoon. Significant warming this week has contributed to instability in the snowpack. Wet snow exists in the top 12″ of snow up to 2500′. Moist snow exists in the top 3-5″ from 2500′ to 3200′.  Temperatures will reach 45ºF over the next few days up to 4000′.

Wet-loose has the potential and likelihood of triggering a wet-slab, increasing the size and consequence of any avalanche. Wet loose are predictable and avoidable. Rollerballs are an indicator that it is time to move to a cooler aspect.

Natural Wet loose natural shown at approx. 1200′ on a S/SE aspect, likely occurred in the afternoon on 4/15. Numerous other wet-loose naturals were observed on the SW aspect of Idaho Peak on 4/16.

Avalanche Problem 3
  • Persistent Slabs
    Persistent Slabs
  • Almost Certain
    Very Likely
    Likely
    Possible
    Unlikely
    Likelihood
  • Historic
    Very Large
    Large
    Small
    Size
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Likelihood of Avalanches
Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches
Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word Size (D scale) Simple Descriptor
Small 1 Unlikely to bury a person
Large 2 Can bury a person
Very Large 3 Can destroy a house
Historic 4 & 5 Can destroy part or all of a village
More info at Avalanche.org

Persistent slabs, 1 to 3 feet deep will be possible to human trigger today at all elevations, on all aspects. Wherever the persistent slab did not fail during the Easter Avalanche Cycle will be suspect with increasing temperatures over the weekend. It will take more force to affect the buried facet/crust weak layer which in some locations is up to 5 feet deep, however it will be more likely in shallower locations on E,SE,S,SW,W aspects. Large cornice fall will also be capable of triggering a persistent slab avalanche.

 

Weather
Sat, April 17th, 2021

The main game changer in the weather this week is the temperature. Since Monday, temps have increased significantly since our sub zero weather a week ago. At 5 am this morning it is 34ºF at 4500′, 32ºF at 3550′, and 35ºF at 2700′. Temps will reach 42-48º F at 1000′ today and 36-44ºF at 3000′. Wind will be NE 5-15 mph at 3000′.

                

NWS Rec Forecast here.

NWS point forecast here.

State Parks Snow Report and Motorized Access information here.

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